One problem which cellular communications systems address is the handing-off of communications between cells. Relative movement between subscriber units and cells causes the subscriber units and the communication links directed thereto to move between cells. To permit continuous communications in an ongoing call, the system must "hand-off" the communication when the subscriber unit crosses a cell boundary. If a communication is not handed off to a new cell upon leaving an old cell, the communication will eventually be lost because the strength of signals over which communications take place would diminish to a point where the satellite cannot receive the subscriber unit's transmissions, or vice versa.
A satellite cellular communication system with non-geosynchronous orbiting satellites must address the motion of the satellite's antenna beam coverage areas relative to the subscriber units. The satellite beams sweep over the earth's surface at such a high rate of speed that a given subscriber will move through a number of beams during a particular call. In many cases, a subscriber unit may also be serviced by two or more different satellites during a call.
Typically, a conventional cellular communication system deals with the satellite motion by "handing-off" subscriber units from beam to beam or satellite to satellite when the subscriber unit detects that better service is available in a new antenna beam. Whenever one of these hand-offs occurs, the subscriber unit is assigned a new communication channel in the new beam. Each of these hand-offs requires the exchange of signaling data between the satellite and the subscriber unit. If these signaling messages are not received in a timely manner, the call may be dropped when the current beam no longer provides enough signal strength to maintain the link.
The hand-off rate increases rapidly as the number of beams is increased to provide more antenna gain, better link performance or a smaller channel reuse distance. In some satellite systems, there are many antenna beams per satellite. As a result, subscriber units may hand-off from beam to beam as often as once per minute and between satellites about once in five minutes. When the number of beams is increased or more satellites are used, the beam to beam hand-off interval may be reduced to 15 seconds or less. The additional processing and bandwidth required to support this increased hand-off rate, along with the loss in call completion reliability creates an undesirable situation, especially with systems using smaller antenna beams.
Thus, what is needed are a method and apparatus that reduces the amount of processing that a subscriber unit is required to perform for handing-off communications between cells.